1. Field of the Invention
The present invention relates to a lens barrel, and more particularly to the structure of a lens barrel in which a lens holding frame is linearly moved in the direction of the optical axis by rotating a cam frame around a fixed frame.
2. Related Art Statement
Hitherto, a variety of cameras of a type having a zoom lens have been suggested and dealt in, and a multiplicity of technological means relating to the zoom lens have been suggested.
An example of the conventional technological means relating to the zoom lens portion will now be described with reference to a technological means disclosed in Japanese Patent Laid-Open No. 2-248909 filed by the applicant of the present invention.
The technological means is an improvement in the structure of the conventional zoom lens barrel composed of a cam ring, a fixed frame and a lens supporting frame and arranged so as to obtain an effect in that the overall length of the zoom lens barrel can be shortened.
Then, the structure of the aforesaid technological means will now be described with reference to FIG. 13.
A first cam ring 102 fitted around a fixed frame 101 has a linear-movement groove 102a. Furthermore, an interlocking cam 108 is integrally fastened to the outer surface of the first cam ring 102. Moreover, a second cam ring 103 is fitted to the inner surface of the fixed frame 101, and a movable frame 104 is fitted to the inner surface of the second cam ring 103. Furthermore, a second-group lens supporting frame 106 and a third-group lens supporting frame 107 are fitted to the inner surface of the movable frame 104. In addition, a first lens group frame 105 holding a first-group lens 116 is fixed to the movable frame 104. A second-group lens frame 117 holding a second group lens 118 is fastened to the second-group lens supporting frame 106 via a shutter unit 119. Moreover, a third-group lens supporting frame 107 holds a third-group lens 120 fixed to the third-group lens supporting frame 107.
On the other hand, a second-group roller (i.e. cam follower) 114 implanted in the second group lens supporting frame 106 is engaged with a linear-movement groove 104a formed in the movable frame 104 and a second-group lens cam groove 103b formed in the second cam ring 103. Furthermore, a third-group roller (i.e. cam follower) 115 implanted in the third-group lens supporting frame 107 is engaged with the linear-movement groove 104b formed in the movable frame 104 and the linear-movement groove 103c formed in the second cam ring 103.
Moreover, a first-group roller (i.e. cam follower) 113 implanted in the movable frame 104 is engaged with both of a first lens cam grove 103a formed in the second cam ring 103 and a linear-movement groove 101d formed in the fixed frame 101. A roller (i.e. cam follower) 112 implanted in the second cam ring 103 is engaged with a lead groove 101c formed in the fixed frame 101 and the linear-movement groove 102a formed in the first cam ring 102.
The operation of the zoom lens barrel thus constituted is performed as follows:
When an interlocking gear 108a is rotated due to rotation of a zooming motor 111 engaged with a reduction gear train 110, the first cam ring 102 is rotated around the optical axis O. As a result, the positional relationship among the linear-movement groove 102a, the lead groove 101c and the roller 112 causes the second cam ring 103 to be moved forward in the direction of the optical axis while being rotated relative to the optical axis. More particularly, rotation of cam ring 102 rotates second cam ring 103 due to the movement of roller 112 along cam grooves 102a and 101c.
Rotation of the second cam ring 103 causes the second cam ring 103 to be moved in the direction of the optical axis due to the movement of first group roller 113 as a result of the rotation of cam ring 103 and the structural relationship among the linear-movement groove 101d, the first-group lens cam groove 103a and the first-group roller 113. Hence, the relationship among the second-group lens cam groove 103b, the linear-movement groove 104a and the second-group roller 114 and that among the third-group lens cam groove 103c, the linear-movement groove 104b and the third-group roller 115 cause the second-group lens supporting frame 106 and the third-group lens supporting frame 107 to also be moved in the direction of the optical axis.
Assuming that the quantities of movements of the first-group lens 116, the second-group lens 118 and the third-group lens 120 respectively are PL1, PL2 and PL3, the optical directional length of the linear-movement groove 102a is PLA, and the optical directional lengths of the first-group lens cam groove 103a, the second-group lens cam groove 103b and the third lens cam groove 103c respectively are PLB, PLC and PLD, the following equations are held: EQU PL1=PLA+PLB EQU PL2=PLA+PLC EQU PL3=PLA+PLD
Assuming that the allowable shortest length of the first cam ring 102 is X3, the allowable shortest length of the second cam ring 103 is X5, the shortest length from the roller 112 to the first-group roller 113 is PLM and the minimum length from the second-group roller 114 to the third-group roller 115 is PLN, the following relationships are held: EQU X3&gt;PLA+PLM EQU X5&gt;MAX (PLB, PLC, PLD)+PLN
Therefore, if the overall length of the lens is made to be the shortest length, PL holds the following relationship: EQU PL&gt;MAX (X3, X5)
Hence, if length PL is the shortest length, the following equation is held: EQU PLA=1/2 (MAX (PL1, PL2, PL3))
Assuming that PL1 is the longest length, the following equation is held: EQU PLA=1/2.times.PL1
Hence, the following relationship is held: EQU PL&gt;1/2.times.PL1+PLM (or PLN)
That is, if the overall length of the lens barrel is the shortest length, length PL is longer than half of the maximum quantity of the movement of a lens group, in which the quantity of the movement of the zoom lens is the largest quantity.
If PLA=1/2 PL1, the following equation is held: EQU PLB=PL1
As a result, the first-group lens cam groove 103a is formed to have the same length as the overall length of the second cam ring 103, causing the first-group lens cam groove 103a to appear outside in a telescope state due to zooming. That is, the lens barrel of the aforesaid type cannot exist.
Therefore, PLA must be considerably larger than 1/2&gt;L1, causing the following relationship to be held: EQU PL&gt;&gt;1/2.times.PL1+PLM (or PLN)
Although a zoom lens barrel of the type structured as described above enables the overall length thereof to be shortened as compared with the prior zoom lens barrel, the shortened quantity has been unsatisfactory. Therefore, the amount of movement of the zoom lens is necessarily increased in the recent circumstance in which the zooming magnification has been enlarged, causing a problem to take place in that the overall length of the lens barrel cannot be shortened.
In order to overcome the aforesaid problem, a technological means has been suggested in Japanese Utility Model Laid-Open No. 2-10515 with which a zoom lens barrel revealing a large magnification while necessitating a short overall length of the lens barrel is provided.
The aforesaid lens barrel has a cam ring comprising an inner helicoid member connected to an outer helicoid member fixed to the fixed frame. Furthermore, the lens barrel has a guide ring capable of rotating around the optical axis with respect to the cam ring, the guide ring being formed integrally with the cam ring in the direction of the optical axis (integrally formed around the optical axis with respect to the fixed frame). A cam groove for moving the lens and a linear-moving groove are respectively formed in the inner surfaces of the aforesaid cam ring and the guide ring. Moreover, a lens-group holding frame having cam followers to be received by the cam ring and the guide ring is fitted to the inner surface of the guide ring.
When the inner helicoid member is rotated around the optical axis in the aforesaid state, the cam ring formed integrally with the inner helicoid member is moved in the direction of the optical axis while being rotated. The aforesaid operation causes the guide ring to be moved forwards or rearwards by the same amount as the corresponding amount of movement of the cam ring, causing the guide ring to be rotated around the optical axis. However it is not rotated with respect to the fixed frame.
Hence, each of the lens-group holding frames is able to be moved in the direction of the optical axis by a length corresponding to the quantity of the movement of the cam ring and corresponding to the optical directional length of the cam ring in the cam groove. As a result, the length of the cam ring and that of the fixed frame can be made shorter than the quantity of the movement of each lens-group holding frame, namely, a movement quantity larger than the optical directional length of the cam ring and that of the fixed frame.
However, the fact that the guide ring can be disposed on only the outer surface of the lens-group holding frame will cause a problem in that the outer diameter of the lens barrel cannot be reduced. What is worse, the quantity of the movement of the cam ring with respect to the fixed frame is limited by the optical directional length of the inner helicoid member, causing the length of the cam groove formed in the cam ring and receiving each lens-group holding frame to be inevitably increased Hence, the length of the cam ring cannot be shortened, causing the overall length of the lens barrel to be increased. As a result, it is as yet difficult to provide a camera having a reduced size and a large zooming magnification.